“The parties to the conflict in Syria have shown time and again that they are willing to take any action or do any deed to secure military advantage, even if it means killing, maiming or besieging civilians into submission in the process.”

He called for protection of life and vital infrastructure, and the ability to safely deliver aid to residents.

WATCH: What if Toronto were Aleppo?

The UN has food at the ready for 150,000 people in western Aleppo, but safe passage must be allowed to the eastern enclave, O’Brien said.

“Nowhere has the cruelty of this war been more grimly witnessed than in Aleppo,” said O’Brien. “Its people have been living in a long, terrifying nightmare reality, which no human being should endure.”

This photo provided by the Syrian Civil Defense White Helmets, which has been authenticated based on its contents and other AP reporting, shows workers pass by bodies after artillery fire struck the Jub al-Quba district in Aleppo, Syria, Wednesday, Nov. 30, 2016.

“For the sake of humanity, we call on — we plead — with the parties and those with influence to do everything in their power to protect civilians and enable aid access to the besieged parts of eastern Aleppo before it becomes one giant graveyard.”

However, Syria and Russia have declined the UN’s request for a pause in the fighting to evacuate 400 sick and wounded in need of immediate treatment, but Russia wants to discuss the idea of setting up four humanitarian corridors, the UN’s Jan Egeland told reporters.

“A humanitarian corridor can work if all the armed actors respect it,” Egeland said.